Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.
If this question can be reworded to fit the rules in the help center, please edit the question.

Part of the reason was to test the effects of such weapons. That's why they chose targets that were relatively untouched thus far.
–
davidjwestMay 2 '12 at 22:43

3

@BrotherJack well by "deploying weapons in Japan" I fisrt think about placing the nuclear weapons on the vessels that were stationed in the Japanese ports against the USSR after the war.
–
AnixxMay 4 '12 at 3:45

2

Probably preventing any significant Soviet occupation of Japan was also a big reason why the US wanted a quick surrender lending impetus to the decision to use the bomb
–
OptJan 11 '13 at 2:01

4

I do not think that by any stretch of the imagination was Japan "ready to surrender". All American pacific operations were met with fierce resistance which increased rather than diminished as they got closer to and started taking home islands. Even after both bombs had been dropped there was still significant resistance to surrender and it was only the direct and unprecedented intervention of Hirohito that changed the balance. Many Japanese would have preferred to see Japan destroyed rather than surrender.
–
PurplePilotSep 16 '13 at 19:12

7 Answers
7

The official reason was to avoid a long and costly battle attempting to force the Japanese to surrender by invading the mainland. The Japanese were tenacious fighters and their tactics of Kamikaze suicide bombers and their courageous defense of their country in engagements such as the Battle of Okinawa, lend substantial credibility to this claim. Some such as General Eisenhower disagreed to as whether such a maneuver was really necessary.

"...in 1945... Secretary of War Stimson, visiting my headquarters in Germany, informed me that our government was preparing to drop an atomic bomb on Japan. I was one of those who felt that there were a number of cogent reasons to question the wisdom of such an act. ...the Secretary, upon giving me the news of the successful bomb test in New Mexico, and of the plan for using it, asked for my reaction, apparently expecting a vigorous assent.

"During his recitation of the relevant facts, I had been conscious of a feeling of depression and so I voiced to him my grave misgivings, first on the basis of my belief that Japan was already defeated and that dropping the bomb was completely unnecessary, and secondly because I thought that our country should avoid shocking world opinion by the use of a weapon whose employment was, I thought, no longer mandatory as a measure to save American lives. It was my belief that Japan was, at that very moment, seeking some way to surrender with a minimum loss of 'face'. The Secretary was deeply perturbed by my attitude..."

Dwight Eisenhower, Mandate For Change, pg. 380

Regardless of whether the Japanese government was seriously considering surrender, the bombing forced the decision and was less costly to the Americans (obviously) than a protracted war. Whether a protracted war would have spared Japanese lives as opposed to several more months of brutal warfare is an open question.

It is my opinion that this motivation was one of several competing reasons as to why the Americans decided to detonate nuclear bombs on Japanese civilian centers. In a large part I feel this was simply the natural evolution of the doctrine of total war applied to aerial bombings of civilian targets, first seen in action during the German bombing of Guernica and continued by the Americans both in the fire bombings of Dresden and Tokyo.

Beyond this there was also the ability to justifiably test an atomic bomb in warfare conditions. American military scientists were very interested in seeing the effects of nuclear weapons in many environments (even detonating them underwater to see what would happen). Obviously the most important environment to test it on would be that of urban or military targets. The prior would be unthinkable, outside of a war anyway.

Also, by deploying the bomb America was able to send a strong message about the balance of power after the war. Given that there is always some degree of uncertainty about political relations and military stability after the war, the bomb was certainly a strong message that Americans were not to be messed with. I don't think any of these motivations were alone sufficient in explaining why the American high command decided to drop the bombs; its far more likely that it was a combination of several of these and possibly additional concerns as well.

The estimated casualty figures for an invasion of Japan were in the millions, lowest I've heard were about a million Americans and 2-3 million Japanese (both killed and wounded). Estimated casualty figures for nuclear bombs (based on known figures for casualties from firebombing cities I believe) were in the order of several tens of thousands per city bombed (the long term effects of radiation sickness were largely unknown and turned out to be less severe than often stated after the war, for decades every death in Hiroshima and Nagasaki due to birth defects and cancer was blamed on the bombs).
–
jwentingFeb 1 '13 at 14:06

2

I think something that is overlooked that we see in spades today's conflicts is that of an active insurgency. Japan, even if it wasn't in a position to effectively fend off an invasion of the island by Allied Forces still had enough spite left in it's military and ruling classes to effectively start and conduct a successful insurgency. Dropping the bomb twice crushed that hope, giving an absolute answer to any idea smacking of resistance. The hidden message could always be touted that if fighting an insurgency was becoming too costly, the Allies could simply leave and nuke the island.
–
danstermeisterFeb 5 '13 at 18:35

2

@mgb-"you can keep doing this for as many years as you wish with no losses." Many years-exactly-and with very dubious results. Military conquest is achieved with boots on the ground. Siege is never more than a preparation for invasion and conquest, and lengthy sieges often end up unsuccessful and are always expensive and difficult to maintain - particulalry at a distance of 5 or 6K miles from the homeland. And besides the military dubiousness of such a siege, USA was tired of war and wanted VICTORY. Politically, a long and expensive siege such as you propose would have been unsustainable.
–
user2590Jul 25 '13 at 7:13

4

@BrotherJack: Not only the "official reason", but the real reason. As to it's necessity, even after two nuclear bombs had been dropped, the Japanese military leadership remained split on the issue of surrender, half preferring to fight on until everyone was dead. Of course the US could not know that before they dropped the bombs, but it shows that their assessment of the Japanese mindset was correct. They would not have surrendered except after a very long and bloody war.
–
Lennart RegebroAug 4 '13 at 14:08

2

@LennartRegebro I haven't seen any info suggesting that half of the Japanese military leadership considered continuing to fight after the dropping of the two bombs. Not to say that your claim isn't true, I'm just not aware of it. Even in that case there is still significant reason to doubt that Japanese morale among the rank & file would allow continuation of the conflict even before the bombs were dropped. All of this is alt. history and open to speculation. Regardless, given a complex geopolitical situation such as this there was certainly more than one simple motivation for the bombing.
–
BrotherJackAug 4 '13 at 16:47

Japan's borders would be restored to those of summer of 1942, requiring the allies to return to Japanese control every island and country that they had been thrown off of, such as Guadalcanal, Iwo Jima, and the Philippines.

Japanese troops would surrender only to Japanese officers. No allied officer or soldier would be allowed to be involved. In effect, the Japanese troops would return to barracks.

The Japanese would pay no reparations to anyone for what they did.

To Westerners, this means that the Japanese would be "returning to barracks" and not surrendering in any way that word was understood to mean. The Allies stated clearly "unconditional surrender"

The Japanese felt that they could disgust and demoralize the Allies with the kamikaze attacks and drive the Allies to the bargaining table.

As for the 2nd atomic bomb, the United States knew what the Japanese were thinking because it was reading their codes. The high command claimed that since it took 4 years for the Allies to make the first atomic bomb, then it would take 4 more years for the second atomic bomb. They knew what an atomic bomb was, and the effects of such weapons, because they had 2 separate atom bomb projects themselves (one in Tokyo using chemical separation of Uranium isotopes, and a diffusion plant in what is now North Korea). Rhodes wrote 2 books (The Making of the Atomic Bomb and Dark Sun) that discussed this along with other national nuclear projects).

A conventional land invasion of the "home islands" of Japan was expected to cost between 500,000 and 1,500,000 Allied troops. Based on how fierce the Okinawa fighting was, and that it took about 25 Japanese casualties to inflict 1 Allied casualty, it would be expected that a conventional land invasion of Kyushu and Honshu would end up having to kill tens of millions of Japanese civilians before they would really surrender.

Cook, in Japan at War, lists the number of soldiers in Japan at the time of surrender ate 4,335,500, with 3,527,000 stationed outside Japan (mostly in China and Korea).

In the end, the allies did not accept any of the conditions, but promised that the status of the emperor would be determined by the Japanese people.

The Japanese actually dropped that as a condition and only asked for guarantees about this. The allies responded that the future of the emperor would be determined by the Japanese people.
–
Lennart RegebroAug 15 '13 at 5:32

"The use of nuclear weapons was not justified by military necessity: the ruling class of the USA pursued political aims - to demonstrate their strength to freedom-loving peoples, to threaten the USSR."

But my opinion is that the bomb was actually developed against Germany and only the fact that they surrendered so quickly saved them and brought such misfortune to Japan. The US already spent much money on the new weapon and just could not leave it unused.

It was also instrumental to demonstrate technical superiority over enemy (including Germany, which at the time was considered the most technologically advanced nation), so that to break an image of "advanced" and "civilized" Germans defeated by barbaric and underdeveloped inferior nations (the Germans did not consider the Anglo-Saxons sub-human, but still always empathized that the Germans are the most productive and creative). It was known that Germany developed a range of "wonder-weapons" so that their enemies had to get something to counter-balance such image.

+1 But, it not only saved American lives, but also Japanese lives.
–
Dan the ManFeb 1 '13 at 16:47

3

@Anixx that's a piece of fine Soviet propaganda, doesn't say anything about the actuality of the situation as it existed at the time.
–
jwentingFeb 5 '13 at 7:08

9

-1 The US already spent much money on the new weapon and just could not leave it unused. Pejorative, subjective, unsubstantiated, and flies directly in the face of the known history of WW2: The USA intended to invade Japan if they refused to surrender. Truman was intent on achieving total victory against Japan. So it was either the Bomb or a million or two American/Japanese lives lost in a costly and bloody invasion. The US's use of the bomb certainly had an intimidating effect on the Soviets, but to contend that was the reason for its use is ludicrous IMO, regardless of any Soviet claims.
–
user2590Jul 24 '13 at 7:08

Why was the bomb built?

This is too big to answer here! Read the great book The making of the atomic bomb. I'll quote it to answer the other questions.

Why the Allied policy of unconditional surrender?

When the Allied leaders met in 1943 at the Casablanca conference, the phrase 'unconditional surrender' was deliberately left out of the joint statement. But Roosevelt later used it in a hasty speech. and Churchill went along unquestioningly, rather than show any tension between the Allies.

In January 1943, Franklin Roosevelt met with Winston Churchill at Casablanca. In the course of the meeting the two leaders discussed what terms of surrender they would eventually insist upon; the word "unconditional"
was discussed but not included in the official joint statement to be read at
the final press conference. Then, on January 24, to Churchill's surprise,
Roosevelt inserted the word ad lib: "Peace can come to the world," the
President read out to the assembled journalists and newsreel cameras,
"only by the total elimination of German and Japanese war power.... The
elimination of German, Japanese and Italian war power means the uncon-
ditional surrender of Germany, Italy, and Japan." Roosevelt later told
Harry Hopkins that the surprising and fateful insertion was a consequence
of the confusion attending his effort to convince French General Henri
Girard to sit down with Free French leader Charles de Gaulle:

We had so much trouble getting those two French generals together that I
thought to myself that this was as difficult as arranging the meeting of Grant
and Lee-and then suddenly the Press Conference was on, and Winston and I
had had no time to prepare for it, and the thought popped into my mind that
they had called Grant "Old Unconditional Surrender," and the next thing I
knew I had said it.

Churchill immediately concurred- "Any divergence between us, even by
omission, would on such an occasion and at such a time have been damag-
ing or even dangerous to our war effort" -and unconditional surrender
became official Allied policy.

Why did the Japanese refuse the Allied terms of surrender?

Why not invade Japan?

Why not ask Russia to help invade Japan?

Why drop atom bombs rather than more fire bombs?

Why not warn Japan about the bomb before dropping it?

Why not demonstrate the bomb on an unpopulated area?

Why was the bomb secret?

Why risk a nuclear arms race when the world sees the bomb?

To Szilard's argument that using the atomic bomb, even testing the
atomic bomb, would be unwise because it would disclose that the weapon
existed, Byrnes took a turn at teaching the physicist a lesson in domestic
politics:

He said we had spent two billion dollars on developing the bomb, and Con-
gress would want to know what we had got for the money spent. He said,
"How would you get Congress to appropriate money for atomic energy re-
search if you do not show results for the money which has been spent al-
ready?"

Why not keep the bomb secret from Russia?

Byrnes' most dangerous misunderstanding from Szilard's point of
view was his reading of the Soviet Union:
>

Byrnes thought that the war would be over in about six months.... He was
concerned about Russia's postwar behavior. Russian troops had moved into
Hungary and Rumania, and Byrnes thought it would be very difficult to per-
suade Russia to withdraw her troops from these countries, that Russia might
be more manageable if impressed by American military might, and that a
demonstration of the bomb might impress Russia. I shared Byrnes' concern
about Russia's throwing around her weight in the postwar period, but I was
completely flabbergasted by the assumption that rattling the bomb might
make Russia more manageable.

You have explained many things... but I don't think you've clearly answered the question. The question did not ask for an exposition regarding the A-bomb's history, it asked about a very specific point regarding the USA's war strategy in using it, one which is not focused upon in this answer. Perhaps some editing is required?
–
user2590Sep 17 '13 at 19:29

The bombing of Japan was a warning to the USSR. The allies knew that Japan would surrender at the drop of a hat without a fight as the had actually asked to be allowed to surrender ten times before the first bomb was dropped.

The official reason for denying them a surrender was that they had placed various requirements on the surrender but in actual fact almost all of these demands were actually accepted in the end - the biggest request from the Japanese PoV being, of course, the continuance of the Emperor's position which was granted.

There was never any chance of a fighting invasion of the Japanese mainland, and everyone knew that well in advance. That idea is simply propaganda. The Japanese were beaten and they knew it. If there was the odd general who wanted to fight on the simple fact was that he would have had to do it himself as the army was on the verge of mutiny, as was the remains of the air force. Again, the vision of the unconquerable samurai who would die before surrendering is a comfortable myth bolstered by a handful of freaks like Hiroo Onoda. Such imagery is no more a true picture of the Japanese army than the Alamo is of the US army en mass.

When MacArthur presented Japan's documentation on the subject of being allowed to surrender, Truman reportedly dismissed the idea without even reading the proposal, commenting that MacArthur was a great general but a lousy politician - a strong clue that the bombing was a political event rather than a military one.

The reason for the second bomb has been debated but is likely to be a combination of two main reasons: firstly to test the second device's design which was substantially different from the first; secondly to hint to Stalin that the US had a supply of these things, not just one made through some super-human effort which would be hard to replicate quickly.

It is easy to forget the degree to which the Japanese had been dehumanised in the US. The idea that a bomb design be tested by being dropped on civilians would not have raised anything like enough of a protest in the US if it had been floated beforehand and, indeed, continued to be regarded as completely justified for decades afterwards in general public opinion.

The ultimate roots of the bombing are a fascinating story of the interaction of militarism and religion on both sides, going back to the days (less than a century before) of Admiral Perry's expedition to "open" (ie, threaten to bombard into submission) Japan and the reaction of the Shogunate to that challenge. In light of that, there is a horrible irony in the fact that Nagasaki was bombed (due to weather), as it was one of the first cities opened to the outside world and was opened specifically as an attempt to avoid Japan being attacked and conquered by the US.

Fascinating set of opinions. For an issue such as this, could you show research?
–
Mark C. WallaceOct 5 '14 at 18:19

Well, MacArthur's attempts to get Truman to listen to the surrender were in 1945, immediately after the bombs had been dropped and officials lifted a reporting restriction link and the fact that their requested terms were granted is a matter of simple record. They had also tried to surrender via the British. Saburo Sakai's (and others) autobiography gives insight into the mess the armed forces were in and their attitude to "no surrender". Japan's relationships with the West are examined in "The invention of religion in Japan" and many others
–
NagoraOct 5 '14 at 19:55

And the rest can be gleaned from the biographies and autobiographies of Western leaders, especially Churchill who was forced into agreeing the plan by FDR's public declaration of "unconditional surrender"; not wanting to show a disunited front, Churchill agreed to the formula and found himself having to flip-flop on the bomb, which he had previously argued was not needed. MacArthur, of course, felt it was pointless. The fact that Japan has no mainland oil supply meant that even if hardliners took over and somehow forced the army to fight, they would starve over the winter. Japan was no threat.
–
NagoraOct 5 '14 at 19:59

The funny thing about it all is how easily all this information is obtainable from primary sources - the people who were involved all wrote about it fairly soon afterwards and unlike the technical information it (mostly) wasn't classified. As I said, the fact that Japan had been trying to surrender was public knowledge before 1946; from 1947 onwards the propaganda machines went into full overdrive and the consequence is that the public probably understand Hiroshima less well today than they did in 1945.
–
NagoraOct 7 '14 at 20:52

The war in Europe had brought the powerful red army, which was mostly responsible for beating Nazis to Berlin in Germany. Us and the soviet Union were having some disagreements on arragements on Europe. Us wanted to let the russians know who is the boss (in the words of Truman) and thus US army roasted nearly half a million Japanese and hurt millions in the coming generations to achieve that. President Truman was instrumental in this by failing to stop the hawks in the army. FDR (Rosevelt would never allow the vision of post war power grab to extend to nuking Japan.) Japanese cities were already incinerated and the most worrisome thing for the Japanese was a Russian Invasion and not the nuclear attack. Also they could not stomach the hanging of their emperor. US delayed assurances to safeguard the emperor until it had exploded the second bomb, on the same day USSR attacked Manchuria, then controled by Japanese forces. Japanese were considered to be sub human at that time and thus the political cost for US politicians was negligable. To understand this well watch Oliver Stone's documentary "Untold history of the USA" episode 2 and 3.

During the Meiji period, Nagasaki became a center of heavy industry.
Its main industry was ship-building, with the dockyards under control
of Mitsubishi Heavy Industries becoming one of the prime contractors
for the Imperial Japanese Navy, and with Nagasaki harbor used as an
anchorage under the control of nearby Sasebo Naval District. These
connections with the military made Nagasaki a major target for bombing
by the Allies in World War II.

https://en.wikipedia.org/wiki/Hiroshima

During World War II, the 2nd General Army and Chugoku Regional Army
were headquartered in Hiroshima, and the Army Marine Headquarters was
located at Ujina port. The city also had large depots of military
supplies, and was a key center for shipping.

Hiroshima was a main military port and main headquarters.

Nagasaki was a city-military-factory, Mitsubishi produced motors of aircrafts.

In large part, Hiroshima and Nagasaki were chosen because most of the bigger cities had already been firebombed.
–
Andrew GrimmJul 25 '13 at 13:17

2

Also, Nagasaki was only the secondary target for the Fat Man raid - the primary target was Kokura.
–
KobuniteJul 25 '13 at 14:52

2Andrew Grimm, partially Hiroshima was bombed by ordinary bomb. 2Kobunite, Kokura was a research's center, it was like DARPA.
–
reticulatusJul 25 '13 at 15:34

@reticulatus doesn't matter. Fact is your explanation of why these targets were chosen is largely off the mark. They were simply the next cities on the target list, period. That list included every city in Japan, as there were military and industrial targets in all of them.
–
jwentingJul 26 '13 at 5:18

1

The rule of war: Assault takes 3 x lives of attackers against 1 life of defenders. Take a approximate number of Japanese soldiers, multiply to three, this number of USA soldiers went back home in the zinc coffin in the scenario, where the USA continue assault of islands and do not touch Japanese Pentagon(Hiroshima). I say again, "Japanese are kamikaze".
–
reticulatusAug 3 '13 at 15:53